One critical aspect of many optical elements is the curvature of the surface of the element. Indeed, refractive lenses derive their ability to converge or diverge light rays from the difference in curvature of their front and rear surfaces. Similarly, the focal plane of curved mirrors is determined by the mirror curvature. For most lenses and mirror applications, the surfaces in question have a spherical shape. One exception to this are lenses for correcting astigmatism which have a curvature that is a combination of spherical and cylindrical surfaces. The key feature of all these elements is that they have two-dimensional surfaces and, therefore, they have two principle curvatures. For example, a flat surface has both curvatures equal to zero. A cylindrical surface has one curvature zero while the other curvature is non-zero. A toroidal surface has both curvatures nonzero and non-equal and sphere may be defined as a special subsection where both curvatures are equal to one another and are non-zero. Accordingly, it will be appreciated that the most general optical elements, are constructed using surfaces for which both curvatures are non-zero and non-equal. Accordingly, those skilled in the art refer to these types of elements as doubly curved. Furthermore, if the curvature also changes along a given direction, the curvature is complex. For the purposes of this application, doubly and complex may be used interchangeably.
The most common optical element is the vision-correction lens used in spectacles. For all but the most severe prescriptions, these lenses are meniscus lenses, in which both surfaces are doubly curved. Corrective lenses may be fabricated having one surface doubly curved and the other flat, but this construction is undesirable since it can lead to undesired optical distortions in addition to esthetic reasons. In addition to vision correction, spectacles with doubly curved lenses are worn to protect the eyes from sunlight, glare, and foreign objects, and as fashion accessories. Other types of eyewear having doubly curved surfaces are goggles, visors, and helmet face plates. Other examples of doubly curved surfaces which light must pass either through or reflect from are windshields, glass block windows, automobile headlamps, skylights, and other optical devices and elements.
For many applications, it is common to coat additional layer or layers onto the surface of an optical element. The layer or layers provide additional functionality, such as light transmission control, anti-reflective properties or scratch resistance. Accordingly, each additional layer acts as an optical element in its own right and when it is attached to another element, the result is a compound element. There are a variety of methods for coating an optical element including vacuum deposition and liquid coating followed by curing. An alternative method is to affix a solid layer to the lens. This provides a cost effective method for achieving the desired functionality. For example, one may create a pair of “mirrored sunglasses” by affixing aluminized Mylar® onto the lenses of an ordinary pair of glasses. Another example is affixing a polarizer to a lens in polarized sunglasses. However, various difficulties arise when attempting to manufacture optical elements where one of these layers has been is attached to a doubly curved surface especially if the layer to be affixed to the doubly curved surface is initially flat. It is quickly seen that unless the initially flat Mylar® is either stretched or cut, it cannot be conformally attached to the doubly curved lens surface. Alternatively, the initially flat layer may be affixed by changing the state of the layer material during the affixing process. If the layer is softened, or even melted and affixed to the state, it can be conformally attached. Obviously, the resulting compound optical element must then be operated at a temperature lower than the temperature at which the layer was affixed.
Although affixing a homogenous solid layers in the manner described above has been accomplished, many more difficulties arise when it is desired to affix multiple layers to an optical element, especially if their thicknesses are to be controlled to better than a micron. Further, this task has been thought impossible if one of the layers affixed is a fluid at the operating temperature. Currently, there are no devices in the market where a fluid with a uniform, micron-sized thickness is affixed to a solid curved layer. If a fluid layer is successfully affixed to a curved surface, a number of applications will become possible. One example of such a device would be a liquid crystal lens based on U.S. Pat. No. 6,239,778, which is incorporated herein by reference.
There have been many unsuccessful attempts to achieve this task since if successful, that would allow use of technologies such as liquid crystals for electronically controllable light transmission. To date, there are no such devices in the market due to the tight tolerances required. One attempt at solving this problem is to employ doubly curved half-lenses which are separated by spacers of the desired gap distance. However, due to the small cell gaps that are required for such devices—on the order of microns—it is difficult to properly align both lenses while maintaining the required gap distance over the entire area of the lenses. The problem is compounded by the presence of electrodes which can result in electrical shorts by micron sized variations in the thickness. Furthermore, if the proper gap spacing is not maintained within less than a micron, the desired optical properties are unattainable. And it is has been found to be quite difficult to properly shape the outer surfaces of such devices so that they conform to the shape of adjacent optical elements. Due to the failure with pre-shaped substrates, the concept of thermoforming a flat multilayered structure where one layer is a fluid has thought to be impossible.
Therefore, there is a need for a device and a method of making it in which two layers are separated from one another by a controlled distance. In other words, this controlled distance provides a gap between the two optical layers and this gap, extending over the area of the optical element, creates an encapsulated volume. This encapsulated volume may be occupied by a fluid substance or substances that perform desired optical, protective, or other functions. If the layers to be affixed to the curved substrate are layers of a liquid crystal device, the resulting compound optical element could, for example, have electronically controllable light transmission. Those skilled in the art will appreciate that maintaining the gap in such devices is critical to ensure correct operation.